Offshore floating platforms are utilized for hydrocarbon extraction and processing. The platforms have tanks that provide the necessary floatation. Water is pumped into at least some of the tanks to provide ballast for positioning the platform at a desired draft. A certain amount of draft may be necessary to prevent capsizing under the effects of wind and waves during storms. The desired draft might be needed both for towing to a well site as well as while stationed at the well site.
Typically, when a platform is being ballasted to the desired draft, it will undergo a region of instability between the initial draft and the desired draft. While in the region of instability, the righting moment of the platform is insufficient to keep the platform upright if it heels excessively. The ballasting must be carefully controlled while in the region of instability to avoid a catastrophe.
Some platforms have a very deep draft, which may be hundreds of feet. Typically, these platforms have a single cylindrical column and may be called “spars” or “deep draft caisson vessels”. Normally, a single column hull is towed to the well site while in a horizontal position, then ballasted to an upright position. These vessels also undergo a region of instability, thus upending the structure at the well site has associated risks. After being upended and ballasted to the desired depth, a catenary mooring system is used to hold the vessel at the well site. A large barge and crane at the well site lifts a deck structure onto the spar after it is at the desired draft and moored.
U.S. Pat. No. 6,371,697 discloses a single column floater that has a larger diameter lower section to provide stability and buoyancy. This patent discloses towing the single column floater to the well site in an upright position. The vessel is towed to the well site at a towing draft, then ballasted at the well site to a desired draft. A catenary mooring system holds the single column floater on station. The deck and structure may be placed on the single column floater while at the dockside, avoiding a need for a barge and crane at the well site. Even though ballasting occurs while the vessel is upright, instability can still exist during the process.
One proposed method to provide stability during ballasting deals specifically with tension leg platforms (“TLP”). A TLP is not moored with a catenary mooring system, rather it is held on station by tendons under tension. The tendons comprise hollow, buoyant strings of pipe extending vertically upward from the sea floor to the platform. Normally the TLP is towed to the well site at a first draft, then ballasted to a second draft. The operator connects the tendons to the TLP and removes ballast to place the tendons in tension. U.S. patent application Publication 2004/0190999 discloses connecting pull-down lines between upper ends of the tendons and pull down devices on the platform. The operator applies tension to the pull-down lines while ballasting to avoid instability. When the tops of the tendons pass through the top terminations on the platform, the operator connects the tendons to the platform, removes the pull-down lines, and deballasts until the desired tension in the tendons is reached.